The present invention relates generally to power storage systems for satellites, and more particularly to a method and apparatus for charging battery cells and controlling the state of charge of the battery cells during charging.
Lithium ion batteries are a desirable source for storing power in various types of systems such as satellite systems. For a large storage system a number of battery cells are typically used together to obtain the desired voltage. Maintaining a predetermined state of charge on the batteries is important to maintain the useful life of the battery.
In FIG. 1, a parallel series single battery bus 10 has a plurality of battery cells 12 coupled together in parallel strings 14. As illustrated, each parallel string has eight cells. Each of the strings 14 is coupled in series. A DC-to-DC converter 16 is coupled in series with the plurality of series connected strings 14. Typically, each string would be provided with its own power supply for topping off the charge therein. One problem with such a configuration is that the number of charge balancing circuits required for such a configuration (one for each cell) increases the cost and mass of the system. In satellite systems in particular, reducing cost and mass is an important priority. Another problem with such a configuration is the number of charge balancing circuits make the system intrinsically less reliable. Also, each of the charge balancing circuits must be tied to a common reference voltage which in practice may be difficult to accomplish.
Referring now to FIG. 2, a series parallel battery bus 20. Serial parallel battery bus 20 includes a plurality of series coupled string 22 of cells 24. Each of the series coupled strings 22 has a DC-to-DC converter that couples each of the string 22 to the common bus 28. The series parallel bus 28 requires an eight-fold increase in DC-to-DC converters. However, due to the redundancy requirements of satellite systems, the total mass of eight smaller converters is significantly greater than the mass of the single converter shown in FIG. 1. Another disadvantage of this approach is that each cell must be equipped with a charge balancing circuit. The electrical connection of eight cells in the parallel sees array of FIG. 1 leads to the requirement for twenty-four circuits. In FIG. 2, series parallel topology requires eight times as many circuits. That is, 192 charge balancing circuits are required in the embodiment of FIG. 2.
Another disadvantage of the parallel series array is apparent in that a short circuit condition in an individual cell may lead to the failure of the entire parallel array. On the other hand, the series parallel array intrinsically provides fault isolation. Because lithium ion technology for satellite systems is relatively new, it may be easy to conclude that for reliability considerations, the series parallel system may be more reliable.
It would therefore be desirable to provide an improved charging circuit that reduces the overall mass and cost of the prior art circuit shown in FIGS. 1 and 2.
The present invention provides a system suitable for multiple cells of a battery system that reduces the overall mass of the system be reducing the number of charge balancing circuits required.
In one aspect of the Invention, a charging circuit has a plurality of parallel strings of series connected battery cells. A plurality of virtual cells are formed In parallel from at least two cells from two different parallel strings. A plurality of charge balancing units are each respectively coupled to one of the plurality of virtual cells. The virtual cell configuration is used when the discharging of the cells is required before recharging to the same state of charge.
In a further aspect of the invention, a method of charging a battery system includes providing a plurality of strings of series connected battery cells, forming a plurality of virtual cells by coupling battery cells in parallel from at least two different strings of the plurality of parallel strings, discharging each of the battery cells in the virtual cells to a predetermined substantially equal state of charge, and charging each of the battery cells.
One advantage of the invention is that the reliability and mass of the overall system is greatly reduced due to the reduced number of charging circuits required. The configuration of the present invention overcomes the major drawbacks of the parallel series in series parallel connections described above in FIGS. 1 and 2.
Other aspects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.